Electronic devices which are capable of music playback have become very popular recently. Such players, especially portable or mobile players, are mainly intended for being used in conjunction with head- or earphones. For a typical mobile use of such devices this is a preferred manner of listening. However, a user may have a large amount or even all of his own music stored on such a device and may also want to use the player as a source of music in other environments. The user may want to play back music with normal loudspeakers.
Additionally, a user may want to use such a player as a source of music or other stored audio data in a vehicle, for example as a replacement for a CD-changer. However, car audio systems usually do not comprise an interface to connect to a mobile player. As one of the main advantages of a mobile music player is the possibility to carry it easily while on the move, a user may want to connect such an audio player to many different playback devices, preferably without the use of cables.
As many audio playback devices like stereo systems and car audio systems comprise an FM (frequency modulation) tuner or receiver, a known implementation of a corresponding wireless transmission is to “mimic” the audio player as a conventional FM radio station and to transmit the audio data encoded as a standard FM radio broadcast transmission.
In the United States of America the FCC (Federal Communications Commission) allows the usage of unlicensed (for example personal/private) FM-radio transmitters according to FCC rule 15 (see section 15.239). Similar legislation applies in many other countries. An unlicensed FM radio transmitter can thus be used for conveniently transmitting sound or music wirelessly from any device, for example from a CD-player or an MP3-player, to an FM radio operating for example in the 88-108 MHz band. In some countries the FM radio band may use a different frequency range. This allows listening to music from such a device for example through a car FM radio. Due to the restricted transmission power with a field strength of 250 μV/m in a distance of 3 meters as in U.S. FCC rule 15, the transmission range of such private transmitters is small. Interference is therefore expected to be low. In addition, interference with licensed FM transmitters, for example a radio station, is usually not allowed.
Thus, unlicensed FM transmitters can be used for example in the car or home environment in order to replay the stored audio content using a car audio or home stereo system.
In addition to music playback devices like MP3-players or CD-players, there are mobile devices that contain an audio player application in addition to a number of further applications like a mobile phone, a camera etc. For example, a number of currently available mobile phone models contain an audio player (for example an MP3 player) and an FM transmitter for transmission of an audio signal to an FM receiver.